In general, it is known that a single-engined rotorcraft possesses measuring instruments that provide the pilot on a continuous basis with values for the main rotor speed and for the engine speed.
The information relating to the main rotor speed is essential for the safety of the rotorcraft. Indeed, a main rotor speed too great would lead to an increase in centrifugal force on the blades, running the catastrophic risk of a blade attachment failing. A main rotor speed too small would lead to a loss of lift. Similarly, the engine speed needs to be monitored in order to remain within the bounds specified by the manufacturer of the rotorcraft, with any change in this engine speed also changing the main rotor speed.
Consequently, engine failure leads to a drop in the main rotor speed which can rapidly lead to a loss of lift for the rotorcraft. The pilot must therefore react immediately, and the only solution available to the pilot being that of reducing the collective pitch of the blades in order to change over to auto-rotation.
Auto-rotation is a technique that makes it possible with a main rotor rotating without drive from the engine, nevertheless to obtain lift that is capable of maintaining the rotorcraft in stabilized downward flight at an acceptable rate of descent. Auto-rotation thus corresponds to safety flight in the event of an engine failure. In order to ensure that the main rotor is put into auto-rotation conditions, the pilot must act immediately at the time of engine failure to reduce the collective pitch of the blades of the main rotor.
Once the collective pitch has been reduced, the pilot merely has to allow the rotorcraft to descend. The relative wind then engages the main rotor in an upward direction, thus enabling the rotor to stabilize on an auto-rotation speed.
In the event of a failure of the engine installation, it can thus readily be understood that it is essential to reduce the pitch of the blades as quickly as possible in order to retain an acceptable main rotor speed.
Nevertheless, starting from hovering flight, pitch reduction needs to be implemented very carefully in order to avoid too great an initial drop in the main rotor speed.
Document FR 2 601 326 discloses a device for switching automatically to auto-rotation flight conditions as soon as a failure appears in the engine installation. That device acts on the flight controls via an automatic pilot system.
Nevertheless, it solves the problem in part only, since some rotorcrafts do not posses an automatic pilot.